Rewrite documentation

Rewrites a majority of our documentation. _I_ think it's better, but do _you_ think it's better? Who knows!

General source of inspiration here would be the documentation for Requests, with a bit of help from django-compressor and a few other libraries as well. 

The setup doc is replaced by a Quickstart doc for getting people up and running much faster, and dumps most of the advanced use cases in the Customization doc. Troubleshooting also got a bit of a reorg to help people find their issues easily. Also includes some extra docs for upgrading from older versions (gleaned from looking at diffs of the setup docs between tags).

I'm pretty happy with the result, but I know that a good thrashing in review will help iron out the wrinkles.

Also, a few other nice things:
- Removes context_processors.py. No idea why that wasn't deleted when we stopped using it. 
- Sets us up to use the ReadTheDocs custom theme when building locally, so we can get a better idea of what the docs will look like.
- Adds CONTRIBUTING.rst to the root of the repo so that it shows up in the Github pull request page, and includes these in the generate docs. Will be nice to be able to point people to standards instead of just notifying them individually of what they did wrong.